1. Field of the Invention
The present invention relates to a radio communication system with localized data transmission mode and distributed data transmission mode, and, in particular, to a method for creating a measurement report to determine conversion of transmission mode and apparatus for using the same.
2. Description of the Related Art
The 3rd Generation Mobile Communication System Partnership Project (referred to as 3GPP) standardization organization has commenced on a Long-term Evolution (referred to as LTE) to existing system criteria. Among numerous physical layer transmission techniques, OFDM (Orthogonal Frequency Division Multiplexing) has become a challenge in all downlink solutions, due to advantages such as higher spectrum utilization efficiency and lower processing complexity.
OFDM is a multi-carrier modulation communication technique, and its basic principle is to divide a high rate data stream into multiple low rate data streams to transmit via a group of orthogonal sub-carriers simultaneously. Because of the features of multi-carrier technique, the OFDM technique bears superior performance in many aspects.
An example of the remarkable superiority of the OFDM technique lies in that Inter-Symbol Interference (ISI) can be completely eliminated when channel delay is less than the length of a Cyclic Prefix (CP), which is introduced in by adding guard spacing to each symbol), for data is transmitted through multiple sub-carriers in parallel and the length of symbol in each sub-carrier is correspondingly increased without sensitivity to channel delay. In this way, each sub-carrier experiences a flat fading channel. An entire OFDM symbol consists of an available OFDM signal and a cyclic prefix. Here, the implementation of the CP is realized by directly copying some samples at the back of the available OFDM signal to the front of the available OFDM signal.
The OFDM technique also bears a high spectrum utilization efficiency. In the frequency domain, OFDM signals overlap actually. This overlap improves spectrum utilization efficiency in great extent.
The OFDM technique also bears a strong ability in anti-narrowband interference or resisting frequency selective fading. Through channel coding and interleaving, the frequency diversity effect and the time diversity effect can be achieved in OFDM so that either narrowband interference or frequency selective fading can be effectively resisted.
Another example of the remarkable superiority of the OFDM technique is that the modulation can be realized through a base-band Inverse Fast Fourier Transform (IFFT), and IFFT/FFT makes available a fast calculation method and can be conveniently implemented in a Digital Signal Processing (DSP) chip and hardware structure.
There are two transmission modes in the OFDM radio transmission system: localized transmission mode and distributed transmission mode.
The called localized data transmission means that the data is transmitted in sequential sub-carriers of the localized sub-band, and the network entity will specify effective modulation and coding method for its data transmission to realize adaptive modulation coding based on channel quality between a Base Station (BS) and User Equipment (UE), thus data transmission throughput is increased. The distributed transmission mode means that the UE uses comb-like subcarriers to transmit over the whole frequency band, making the sub-carriers that transmit data be distributed in the whole frequency band as much as possible, thus maximizing frequency diversity gain. Usually the localized data transmission mode which can use adaptive modulation coding and frequency scheduling has greater transmission gain. However, for some channels which change very fast, channel conditions predicted at some time can't reflect that of the next time, therefore localized transmission is hard to implement. In this condition, distributed data transmission is often used to transmit data via frequency diversity gain.
The process of transmission for downlink data is now described.
For a localized transmission mode, a UE will measure the channel quality for each sub-band in the system frequency band, and then report the measured Channel Quality Indicator (CQI) of each sub-band to the BS. Having received the CQIs, the BS determines whether to allocate frequency resources to the UE, and which localized frequency bands will be allocated to UE for data transmission, according to the reported CQI from each UE and the system current load. During the process of data transmission, it is necessary for the UE to continue to measure the channel quality for each sub-band in the system frequency band and transmit the measured CQI to the base station so as to achieve the goals of frequency scheduling and Adaptive Modulation Coding (AMC) for data transmission. In this way, maximum throughput of data transmission may be reached.
For a distributed transmission mode, the UE will measure the mean channel quality of the entire system frequency band, then report the single average CQI to the base station. Having received this mean CQI, the BS determines whether to allocate distributed frequency resources to the UE according to the reported CQI from each UE and the system current load. During the process of data transmission, it is also necessary for the UE to continue to measure the average CQI of the entire system frequency band and transmit the measured CQI to the base station so as to achieve the goals of frequency scheduling and Adaptive Modulation Coding (AMC) for data transmission. In this way, maximum throughput of data transmission can be reached.
It is seen from the descriptions above that, when in the localized transmission mode, the contents of the measurement report sent to the BS from the UE are CQIs in sub-bands, and, when in the distributed transmission mode, the UE only sends the single average CQI to the BS. Therefore, the number of bits necessary for the transmission of CQI information in the localized transmission mode is far greater than that in the distributed transmission mode.
In the LTE system, the system will select an appropriate transmission mode for data transmission between the BS and the UE based on the channel condition between them. The conversion between localized transmission and distributed transmission will probably happen when the channel condition between the BS and the UE changes. OFDM transmission is currently used in IEEE 802.16E radio transmission technology specification. The transition between the two modes will probably happen during the data transmission process, with the conversion process as follows. For a distributed transmission mode to a localized transmission mode, the conversion process is a follows. If, in a specific time duration, the maximum value of standard deviation of Signal to Noise Radio (SNR) measured in all frequency sub-bands in the time domain is lower than an predefined threshold value and the mean SNR of the whole frequency band is larger than the predefined threshold value, then the UE transmits transition request from the distributed transmission mode to the localized transmission mode to the BS, and CQIs of 5 sub-bands with best channel quality. When the BS receives such a request, it specifies sub-bands that the UE may use and appropriate adaptive modulation coding based on channel quality in several sub-band reported by the UE. In this way the UE may transit its data transmission mode from distributed transmission to localized data transmission.
For a localized transmission mode to a distributed transmission mode, the conversion process is as follows. If, in a specific duration, the maximum value of standard deviation of SNRs measured in all frequency sub-bands in time domain is higher than a predefined threshold value, then the UE transmits a transition request from the localized transmission mode to the distributed transmission mode to the BS, and sends the channel quality of the whole frequency band until the BS has allocated distributed channel resource for the UE. When the UE receives a distributed channel resource indicator, it may transit its data transmission mode from localized data transmission to distributed data transmission.
It may be seen from a data transmission mode conversion method in the IEEE 802.16E specification that if the maximum value of standard deviation of SNRs measured in all frequency sub-bands in the time domain is comparatively large, the distributed transmission mode is suitable for the UE, and if the maximum value of standard deviation of SNRs measured in all frequency sub-bands in time the domain is comparatively small and the mean SNR of the whole frequency band is larger than a predefined threshold value, the localized transmission mode is the suitable transmission mode for the UE. As the time domain channel variation for a high speed moving UE has a comparatively large variance, and the time domain channel variation for a low speed moving UE has a comparatively small variance, according to the IEEE 802.16E specification, distributed data transmission is suitable for UE's moving at high speed. For UE's moving at low speed, if the mean SNR of its whole frequency band is high, then localized data transmission is suitable.
As for localized transmission mode, since more uplink signaling is needed to transmit CQI in several sub-bands, the corresponding uplink signaling load is heavy. However, for data transmission in a flat fading channel condition, localized data transmission can't acquire much selective frequency gain compared to distributed data transmission, because SNRs of all sub-carriers are basically the same no matter that they are localized data transmission carriers or distributed data transmission carriers. Thus, their adaptive modulation coding gains are basically the same regardless of whether they use distributed data transmission or localized data transmission. But localized data transmission needs an uplink CQI report for multi-bands, thus introducing heavier signaling load compared to distributed data transmission. It may be seen that for flat fading channel, even for low speed UEs with a comparatively high mean SNR, distributed data transmission may still acquire the same selective frequency gain as that of localized data transmission, and its uplink signaling overhead is small. Thus, the distributed data transmission mode is more suitable.
Therefore some improvements may be made for the mode transition method of the IEEE 802.16E specification to make the UE use a more suitable transmission mode to transmit data.